In the veterinary field, the use of sustained release dosage forms of medication which remain in the rumeno reticular sac for a prolonged period of time is well known. These dosage forms are usually in the form of boluses and are dense and heavy enough to stop in the rumeno reticular sac and remain there rather than pass into the alimentary tract and be eliminated intact. The physical form of these dosage forms is retained over a long period of time in the sac while the therapeutically active substance is slowly released by erosive or solubilization action within the sac. In other words, regardless of the therapeutically active substance to be administered, the bolus itself must comply with the physical requirements as to density weight. In order to meet the proper requirements for density and weight, iron, sand or any other relatively high density matrix may be employed in the preparation of the bolus. Exemplary of such prior art practices are U.S. Pat. No. 3,507,952 and U.S. Pat. No. 4,564,363.
European Patent Application 0,164,927 discloses a release device for ruminants comprising a succession of annular medicament containing segments contained in a liquid impermeable casing exposed at one end. The segments are penetrated by a rod of degradable material such as magnesium alloy. The rate of administration or bioavailability of each segment depends on the speed of degradation of the magnesium rod.
A major problem associated with the use of boluses in animals is the frequency that the boluses must be administered and the increase labor required to continually administer these boluses to the animal. Further, most ruminants such as cattle, sheep or goats are grazing animals which are prone to wander over vast areas of land for an extended period of time, thus making repeat administration of these boluses to the animals extremely difficult and economically unfeasible.